Harriet9 Posted January 3, 2021 Report Share Posted January 3, 2021 Hi, I've been firing unglazed vitrified, colored pieces at cone 4 . The colors been coming out great for a year or so but now they’ve started to have white marks on them and a rougher than normal. Any thoughts on why this would happen? Thanks in advance! Link to comment Share on other sites More sharing options...
Sorcery Posted January 4, 2021 Report Share Posted January 4, 2021 Pics (conveniently resized by sending to a confused spouse via text) would help. Sorce Link to comment Share on other sites More sharing options...
Harriet9 Posted January 4, 2021 Author Report Share Posted January 4, 2021 Thank you! I added more water to my slip this time and I think this is probably what’s causing it but would love any thoughts. Picture added, thank you. Link to comment Share on other sites More sharing options...
Min Posted January 4, 2021 Report Share Posted January 4, 2021 Could you give more details on your process and claybody? Slipcast cone 4? Looks like sponge marks? Change in water supply or recipes? Verified firing with cones? Link to comment Share on other sites More sharing options...
Harriet9 Posted January 4, 2021 Author Report Share Posted January 4, 2021 Sure, thanks. I mix the slip from dry ingredients and actually fire it to cone 3 normally with a 10min hold but this was 15 to help improve the vitrification, this is a vitrified earthenware body. I fire a low bisque and then clean and polish the pieces and then fire up to temperature. They normally come out very smooth with an even color. I was upping the quantity of slip and had increased the amount of water more than the normal ratio so am thinking this may account for the marks. I’m using the same water source as previous. I’m thinking I’ll go back to my old recipe and remix the colors and see how they look and also maybe try firing some with the 10min hold? There’s also more warping than normal which I presume is also the water. I appreciate any help, thank you! Link to comment Share on other sites More sharing options...
Min Posted January 5, 2021 Report Share Posted January 5, 2021 A few thoughts in no particular order - Do you measure the specific gravity and viscosity of the slip? Adding excess water is obviously going to mess up the specific gravity so I'ld let some evaporate off until the specific gravity gets back to what it should be. If you are mixing the slip body yourself are you using the same bags of materials or has one of them changed? Same stains? Any chance of an error with the weighing of the last batch? Have you verified you are reaching your target cone with witness cones? 3 hours ago, Harriet9 said: to help improve the vitrification, this is a vitrified earthenware body I know in England there are some earthenware bodies that actually fire up to cone 6 and what we would call midrange in North America, typically we don't see vitrification of earthenware bodies though. Mature yes but not vitrified. Semantics perhaps. What is the absorption percentage of your body? Link to comment Share on other sites More sharing options...
Harriet9 Posted January 5, 2021 Author Report Share Posted January 5, 2021 Thanks so much! I don’t measure the viscosity, just by sight and how it pours. I mix quite a few different colors with different percentages and the last few I mixed were all with more water that seems to be the common denominator. I’ll also start using distilled water as that may be contributing. This is an English vitrified cone 4 recipe but like you said it’s not technically vitrified but doesn’t absorb water and has a glassy sheen. I was getting too much warping at cone 4 so that’s why I do cone 3 with a hold which has worked well. I don’t know what the absorption percentage is. Link to comment Share on other sites More sharing options...
Min Posted January 5, 2021 Report Share Posted January 5, 2021 Do you add barium carb to prevent soluble salts causing efflorescence? Recipe? Link to comment Share on other sites More sharing options...
liambesaw Posted January 5, 2021 Report Share Posted January 5, 2021 If it's older slip and has nepheline syenite as the feldspar it could be that too much sodium has left the neph sy and is coming out as it dries. Just a shot in the dark since I have no idea what the recipe is. Also are you using sodium silicate and soda ash as the deflocculant? If so try switching to a synthetic deflocculant like darvan-7. To me it looks like scumming, what @Minis referring to as efflorescence. Its when the clay dries and a thin layer of salt forms on the surface. It can cause splotchiness like this especially at lower temperatures. This is why I'm asking about soluble sodium which seems to be the biggest offender. Link to comment Share on other sites More sharing options...
Babs Posted January 5, 2021 Report Share Posted January 5, 2021 With@Min on yhis. What colour is your clay body? When fired without slip does it scum? I had a batch of clay which did not scum anf then next batch scummed...producer wary of Barium talk? Not belittling Barium talk here! Link to comment Share on other sites More sharing options...
Harriet9 Posted January 5, 2021 Author Report Share Posted January 5, 2021 Thanks so much! I don’t measure the viscosity, just by sight and how it pours. I mix quite a few different colors with different percentages and the last few I mixed were all with more water that seems to be the common denominator. I’ll also start using distilled water as that may be contributing. This is an English vitrified cone 4 recipe but like you said it’s not technically vitrified but doesn’t absorb water and has a glassy sheen. I was getting too much warping at cone 4 so that’s why I do cone 3 with a hold which has worked well. I don’t know what the absorption percentage is. Link to comment Share on other sites More sharing options...
Harriet9 Posted January 5, 2021 Author Report Share Posted January 5, 2021 @min, I haven’t been adding barium carbonate as this is the first time this has happened in 2 years of no issues. Will go back to my old recipe and see if it does the same, if so will try bc. Tiny amounts right? I’m also nervous about using it as it’s very toxic I believe. @Babs it hasn’t scummed either without the color. The only difference is the amount of water I’ve added which I believe can cause salt residue also. @liambesawthis is a new batch, my recipe does have a lot of neph sy but this a new problem as I mentioned. There was also more warping which I’m thinking may be the extra water. Really appreciate everyone’s thoughts. I feel like I’m always learning! Link to comment Share on other sites More sharing options...
liambesaw Posted January 5, 2021 Report Share Posted January 5, 2021 Just curious since it is cold this time of year. What temperature is your slip kept? I ask because you can run into more issues the colder it gets, I believe the optimal temperature for slipcasting is around 80⁰f? Paging @glazenerdi think I remember him expounding on slip/temperature interaction... I know it can tend to swirl more when it's colder. Link to comment Share on other sites More sharing options...
Harriet9 Posted January 5, 2021 Author Report Share Posted January 5, 2021 Oh that’s interesting. I live in LA so it’s pretty temperate but it has been cold. I work outside, it’s been around 65ish lately. Also interested if anyone else feels it’s the extra water. Thanks! Link to comment Share on other sites More sharing options...
glazenerd Posted January 6, 2021 Report Share Posted January 6, 2021 2 hours ago, liambesaw said: Just curious since it is cold this time of year. What temperature is your slip kept? I ask because you can run into more issues the colder it gets, I believe the optimal temperature for slipcasting is around 80⁰f? Paging @glazenerdi think I remember him expounding on slip/temperature interaction... I know it can tend to swirl more when it's colder. D.D. Buttons (Alfred U) did extensive studies on the effects of temperature on particle distribution. At 68F, the negative particle charge on clay particles drops by 1/3rd. As the temperature continues to drop: so does the particle charge. Sodium Silicate and Darvan work (in part) by imparting a strong negative charge which suspends the particles in water. The water carries that charge: and as temperature drops; it effects suspension. When particles drop out of suspension in an irregular pattern: it weakens the form. Stain can also drop out in an irregular fashion. I do think Min's comments on scumming is lso applicable: blotchy color pattern suggests that as well. Tom side note- nice bird bath Liam Link to comment Share on other sites More sharing options...
Min Posted January 6, 2021 Report Share Posted January 6, 2021 4 hours ago, Harriet9 said: I haven’t been adding barium carbonate as this is the first time this has happened in 2 years of no issues. Will go back to my old recipe and see if it does the same, if so will try bc. Tiny amounts right? I'ld try 0.30% of the dry weight of materials. (thats zero point three), if this isn't enough then bump it up to 0.35% (zero point three five) Don't eyeball or guesstimate this. 4 hours ago, Harriet9 said: There was also more warping which I’m thinking may be the extra water. I think what happens when the specific gravity is too low (ie an excess of water) is the walls not only take longer to cast but they won't be as dense. This could very well lead to more warpage, this also could be why they feel rougher than previously. It's also going to age your moulds faster and increase the drying shrinkage of your pots. I would really suggest getting your specific gravity dialled in. Link to comment Share on other sites More sharing options...
Harriet9 Posted January 6, 2021 Author Report Share Posted January 6, 2021 Thanks @Min I’ll try that if the problem persists, super helpful. Link to comment Share on other sites More sharing options...
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